What is PurpleEventCallback?
Project Purple was the codename for iOS back when it was basically a skunkworks project within Apple. This callback is basically just an event bridge between the CoreFoundation layer (ported from OS X) and the UIKit layer (the next-generation Cocoa framework).
Just one more piece of information on PurpleEventCallback.
It's part of Graphics Services. In the case, if you disassemble GraphicsServices framework, you will find the source code of this method.